Chloric acid - alkali metal chlorate mixtures and chlorine dioxide generation

ABSTRACT

An aqueous solution of chloric acid and alkali metal chlorate is produced in an electrolytic cell having an anode compartment, a cathode compartment, and at least one ion exchange compartment between the anode compartment and the cathode compartment. The process comprises feeding an aqueous solution of an alkali metal chlorate to the ion exchange compartment, electrolyzing an anolyte in the anode compartment to generate hydrogen ions, passing the hydrogen ions from the anode compartment through a cation exchange membrane into the ion exchange compartment to displace alkali metal ions and produce an aqueous solution of chloric acid and alkali metal chlorate, passing alkali metal ions from the ion exchange compartment into the cathode compartment, removing the aqueous solution of chloric acid and alkali metal chlorate from the ion exchange compartment, and, increasing the chlorate ion concentration to provide the aqueous solution of chloric acid and alkali metal chlorate with a total chlorate ion to water molar ratio of about 0.15 or greater.

This is a Continuation-in-part application of U.S. Ser. No. 07/475,603 filed Feb. 6, 1990 now U.S. Pat. No. 5,084,148.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for electrochemically Producing chloric acid-alkali metal chlorate solutions. More Particularly, this invention relates to the electrochemical production of chloric acid-alkali metal chlorate solutions suitable for the generation of chlorine dioxide.

2. Brief Description of the Prior Art

Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses due to its high oxidizing power. There is a variety of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators employed, for example, in pulp and paper production, utilize an alkali metal chlorate salt, a reducing agent, and an acid in a chemical process for producing chlorine dioxide. These generators and the processes employed also produce by-product salts such as sodium chloride, sodium sulfate, or sodium bisulfate. In pulp and paper mills, the typical by-product is sodium sulfate (saltcake) which is converted into a sulfur salt of sodium in a high temperature boiler and used in the paper process. Boilers require energy and the paper mills have a limited boiler capacity. Increasing the production of chlorine dioxide generally means increased capital investment to provide the added boiler capacity required to process the added amounts of saltcake by-product produced.

Thus a process which reduces the amount of a by-product salt, such as sodium chloride or sodium sulfate, produced while efficiently generating chlorine dioxide is commercially desireable.

U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumberger teaches a process for producing chloric acid by passing an aqueous solution containing from 0.2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic exchange resin at a temperature from 5° to 40° C. The process produces an aqueous solution containing from 0.2 gram mole to about 4.0 gram moles of HClO₃. This process requires the regeneration of the cationic exchange resin with acid to remove the alkali metal ions and the treatment or disposal of the acidic salt solution.

K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989, describe a process for chlorine dioxide which electrolyzes a chloric acid solution produced by passing an aqueous solution of an alkali metal chlorate through an ion exchange resin. The electrolyzed solution contains a mixture of chlorine dioxide and chloric acid which is fed to an extractor in which the chlorine dioxide is stripped off. The ion exchange resin is regenerated with hydrochloric acid and an acidic solution of an alkali metal chloride formed.

In U.S. Pat. No. 4,683,039, Twardowski et al describe a method for producing chlorine dioxide in which the chlorine dioxide is produced in a generator by the reaction of sodium chlorate and hydrochloric acid. After separating chlorine dioxide gas, the remaining sodium chloride solution is fed to a three-compartment cell to form sodium hydroxide and an acidified liquor which is returned to the chlorine dioxide generator.

Each of the above processes produces a fixed amount and type of by-product salt.

M. Lipsztajn et al teach an electrolytic-dialytic process for producing chloric acid and sodium hydroxide from sodium chlorate. Chlorate ions are transferred through an anion-exchange membrane and sodium ions are passed through a cation-exchange membrane (U.S. Pat. No. 4,915,927, Apr.10, 1990).

BRIEF SUMMARY OF THE INVENTION

Now a process has been discovered which permits variability in the composition of a chlorate solution used in chlorine dioxide generators. Further, the process permits a reduction in the amount of acid required and subsequently the amount of salt by-product produced in the chlorine dioxide generator. Still further, the process allows for the production of an alkali metal hydroxide as a valuable by-product or acidic solutions of alkali metal salts at reduced energy costs. In addition, the process results in the reduction of process steps and process equipment required for the production of chlorine dioxide.

These and other advantages are accomplished in a process for electrolytically producing an aqueous solution of chloric acid and alkali metal chlorate in an electrolytic cell having an anode compartment, a cathode compartment, and at least one ion exchange compartment between the anode compartment and the cathode compartment, characterized by feeding an aqueous solution of an alkali metal chlorate to the ion exchange compartment, electrolyzing an anolyte in the anode compartment to generate hydrogen ions, passing the hydrogen ions from the anode compartment through a cation exchange membrane into the ion exchange compartment to displace alkali metal ions and produce an aqueous solution of chloric acid and alkali metal chlorate, passing alkali metal ions from the ion exchange compartment into the cathode compartment, removing the aqueous solution of chloric acid and alkali metal chlorate from the ion exchange compartment, and, increasing the chlorate ion concentration to provide the aqueous solution of chloric acid and alkali metal chlorate with a total chlorate ion to water ratio of about 0.15 or greater.

BRIEF DESCRIPTION OF DRAWINGS

More in detail, the novel process of the present invention and its application in producing chlorine dioxide can be carried out in apparatus illustrated in the following FIGURES.

FIG. 1 is a sectional side elevational view of an electrolytic cell which can be employed in the novel process of the invention.

FIG. 2 is a sectional side elevational view of an additional electrolytic cell which can be employed in the novel process of the invention.

FIG. 3 is a diagrammatic illustration of a system which can be employed in the process of the invention.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1 shows an electrolytic cell 4 divided into anode compartment 10, ion exchange compartment 20, and cathode compartment 30 by cation permeable ion exchange membranes 16 and 24. Anode compartment 10 includes anode 12, and anode spacer 14. Anode spacer 14 positions porous anode 12 with respect to cation permeable ion exchange membrane 16 and aids in the disengagement of anolyte gas produced. Anolyte disengager 18 completes the disengagement of anolyte gas from the spent anolyte solution. Ion exchange compartment 20 includes spacer material 22 which provides a flow channel between cation permeable ion exchange membranes 16 and 24 for the aqueous alkali metal chlorate solution. Cathode compartment 30 includes cathode 32, and cathode spacer 34. Cathode spacer 34 positions cathode 32 with respect to cation permeable ion exchange membrane 24 and aids in the disengagement of catholyte gas produced. The disengagement of catholyte gas from the spent catholyte solution is accomplished in cathode disengager 36.

In FIG. 2, electrolytic cell 4 has been expanded to include a second ion exchange compartment 40 which is positioned between anode compartment 10 and ion exchange compartment 20. Cation permeable ion exchange membrane 42 separates anode compartment 10 from ion exchange compartment 40. The sodium chlorate feed solution enters the lower part of ion exchange compartment 20, flows upward and out of ion exchange compartment 20 into the upper part of ion exchange compartment 40. The HClO₃ /NaClO₃ product solution is recovered from the lower part of ion exchange compartment 40.

The flow direction in the ion exchange compartments can also be reversed, for example, with the solution from the top of ion exchange compartment 20 being fed to the bottom of ion exchange compartment 40. The product solution then exits from the top of ion exchange compartment 40.

In the process depicted in FIG. 3, the HClO₃ /NaClO₃ solution from ion exchange compartment 20 of electrolytic cell 4 is fed to concentrator 50. Water is removed from the solution by evaporation under a vacuum and a slurry of NaClO₃ crystals in a HClO₃ /NaClO₃ solution is produced. The slurry is fed to separator 52 where the NaClO₃ crystals are removed from a concentrated solution of chloric acid and sodium chlorate as the product. Sodium chlorate crystals from separator 52 are recycled through line 53 to dissolving tank 54.

DETAILED DESCRIPTION OF INVENTION

An aqueous solution of an alkali metal chlorate is fed to the single or multiple ion exchange compartments of the electrolytic cell. Suitable alkali metal chlorates include sodium chlorate, potassium chlorate and lithium chlorate. In order to simplify the disclosure, the process of the invention will be described using sodium chlorate, which is a preferred embodiment of the alkali metal chlorates. As shown in FIG. 3, the sodium chlorate feed solution may be prepared, for example, by dissolving crystalline sodium chlorate in water. Commercial sodium chlorate is suitable as it has a low sodium chloride content and the formation of undesireable amounts of chlorine dioxide in the electrolytic cell is prevented.

Aqueous sodium chlorate feed solutions which may be employed contain any suitable concentrations of sodium chlorate up to about saturation at the temperatures at which the solutions are fed to the ion exchange compartment. For example, solutions having a concentration in the range of from about 1% by weight to about 60% by weight of NaClO₃, preferably from about 20 to about 55%, and more preferably at from about 30 to about 50% by weight of NaClO₃, at temperatures in the range of from about 0° to about 100°, and preferably from about 15° to about 80° C.

The novel process of the invention utilizes an electrochemical cell to generate hydrogen ions that displace or replace a portion of the sodium ions present in the aqueous sodium chlorate solution feed stream.

The generation of hydrogen ions in the process of the present invention in the anode compartment is accompanied, for example, by the oxidation of water on the anode into oxygen gas and H+ ions by the electrode reaction as follows:

    (4) 2H.sub.2 O→O.sub.2 +4H.sup.+ +4e.sup.-

The anode compartment contains an anolyte, which can be an aqueous solution of any non-oxidizable acid electrolyte which is suitable for conducting hydrogen ions into the ion exchange compartment. Non-oxidizable acids which may be used include sulfuric acid, phosphoric acid, perchloric acid and the like. Where a non-oxidizable acid solution is used as the anolyte, the concentration of the anolyte is preferably selected to match the osmotic concentration characteristics of the alkali metal chlorate solution fed to the ion exchange compartment to minimize water exchange between the anode compartment and the ion exchange compartment. Additionally, a solution of hydrochloric acid or a mixture of hydrochloric acid and an alkali metal choride can be used as the anolyte, which results in a generation of chlorine gas at the anode. Where a chlorine generating anolyte is employed, it is necessary to select a cation exchange membrane as the separator between the anode compartment from the ion exchange compartment which is stable to chlorine gas.

The anode compartment may also employ as the anolyte electrolyte a strong acid cation exchange resin in the hydrogen form and an aqueous solution such as deionized water.

Any suitable anode may be employed in the anode compartment, including those which are available commercially as dimensionally stable anodes. Preferably, an anode is selected which will generate oxygen gas.

These anodes include porous or high surface area anodes. As materials of construction for the anodes metals including platinum, gold, palladium, or mixtures or alloys thereof, or thin coatings of such materials on various substrates such as valve metals, i.e. titanium, can be used. Additionally oxides of iridium, rhodium or ruthenium, and alloys with other platinum group or precious metals metals could also be employed. Commercially available oxygen evolution anodes of this type include those manufactured by Englehard (PMCA 1500) or Eltech (TIR-2000). Other suitable anode materials include graphite, graphite felt, a multiple layered graphite cloth, a graphite cloth weave, carbon, etc.

The hydrogen ions generated in the anode compartment pass through the cation exchange membrane into the sodium chlorate solution in the ion exchange compartment. As a hydrogen ion enters the solution, a sodium ion is displaced and by electrical ion mass action passes through the cation membrane adjacent to the cathode compartment to maintain electrical neutrality.

The novel process of the invention as operated results in the conversion of sodium chlorate to chloric acid over a wide range, for example, from about 1 to about 99.9%, preferably from about 5 to about 95, and more preferably from about 15 to about 90%.

The sodium chlorate feed solution concentration, the residence time in the ion exchange compartment as well as the cell amperage are factors that affect the extent of the conversion of sodium chlorate to chloric acid.

Using very dilute solutions of sodium chlorate, high percentages of conversion of NaClO₃ to chloric acid can be achieved, i.e. up to 99.9% conversion. For a single pass flow through system, typical residence times in the ion exchange compartment are between about 0.1 to about 120 minutes, with a more preferred range of about 0.5 to about 60 minutes.

Thus the concentration of sodium chlorate in the solution fed to the ion exchange compartment and the flow rate of the solution through the ion exchange compartment are not critical and broad ranges can be selected for each of these parameters.

The novel process of the present invention is operated at a current density of from about 0.01 KA/m² to about 10 KA/m², with a more preferred range of about 0.05 KA/m² to about 3 KA/m².

Current efficiencies during operation of the process of the invention can be increased by employing additional ion exchange compartments, as illustrated by FIG. 2, which are adjacent and operated in a series flow pattern.

Adjusting the width of the ion exchange compartment can also alter the operating cell voltage and current efficiency. The width, or space between the cation exchange membranes forming the walls of the ion exchange compartment, is in the range of from about 0.1 to about 10, and preferably from about 0.3 to about 5 centimeters.

In an alternate embodiment the ion exchange compartment contains a cation exchange medium. Cation exchange mediums which can be used in the ion exchange compartment include cation exchange resins. Suitable cation exchange resins include those having substrates and backbones of polystyrene based with divinyl benzene, cellulose based, fluorocarbon based, synthetic polymeric types and the like. Where more than one ion exchange compartment is employed, inclusion of the cation exchange medium is optional for each compartment.

Functional cationic groups on these mediums which may be employed include carboxylic acid, sulfonic or sulfuric acids, and acids of phosphorus such as phosphonous, phosphonic or phosphoric. The cation exchange resins are suitably ionically conductive so that a practical amount of current can be passed between the cation exchange membranes used as separators. Various percentage mixture of resins in the hydrogen form and the sodium form may be used in various sections of the ion exchange compartments on assembly to compensate for the swelling and contraction of resins during cell operation. For example, percentage ratios of hydrogen form to sodium form may include those from 50 to 100%.

The use of cation exchange resins in the ion exchange compartment can serve as an active mediator which can exchange or absorb sodium ions and release hydrogen ions. The hydrogen ions generated at the anode thus regenerate the resin to the hydrogen form, releasing sodium ions to pass lo into the cathode compartment. Their employment is particularly beneficial when feeding dilute sodium chlorate solutions as they help reduce the cell voltage and increase conversion efficiency.

Preferred as cation exchange mediums are strong acid type cation exchange resins in the hydrogen form as exemplified by low cross-linked resins such as AMBERLITE® IRC-118 (Rohm and Haas Co.) as well as higher cross-linked resins i.e. AMBERLITE® IRC-120. High surface area macro-reticular or microporous type ion exchange resins having sufficient ionic conductivity in the ion exchange compartments are also suitable.

Physical forms of the cation exchange resin which can be used are those which can be packed into compartments and include beads, rods, fibers or a cast form with internal flow channels. Bead forms of the resin are preferred.

Cation exchange membranes selected as separators between compartments are those which are inert membranes, and are substantially impervious to the hydrodynamic flow of the alkali metal chlorate solution or the electrolytes and the passage of any gas products produced in the anode or cathode compartments. Cation exchange membranes are well-known to contain fixed anionic groups that permit intrusion and exchange of cations, and exclude anions from an external source. Generally the resinous membrane or diaphragm has as a matrix, a cross-linked polymer, to which are attached charged radicals such as --SO₃ ⁻ and/or mixtures thereof with --COOH⁻. The resins which can be used to produce the membranes include, for example, fluorocarbons, vinyl compounds, polyolefins, hydrocarbons, and copolymers thereof. Preferred are cation exchange membranes such as those comprised of fluorocarbon polymers or vinyl compounds such as divinyl benzene having a plurality of pendant sulfonic acid groups or carboxylic acid groups or mixtures of sulfonic acid groups and carboxylic acid groups. The terms "sulfonic acid group" and "carboxylic acid groups" are meant to include salts of sulfonic acid or salts of carboxylic acid groups by processes such as hydrolysis.

Suitable cation exchange membranes are readily available, being sold commercially, for example, by Ionics, Inc., Sybron, by E. I. DuPont de Nemours & Co., Inc., under the trademark "NAFION®", by the Asahi Chemical Company under the trademark "ACIPLEX®", and by Tokuyama Soda Co., under the trademark "NEOSEPTA®". Among these are the perfluorinated sulfonic acid type membranes which are resistant to oxidation and high temperatures such as DuPont NAFION® types 117, 417, 423, etc., membranes from the assignee of U.S. Pat. No. 4,470,888, and other polytetrafluorethylene based membranes with sulfonic acid groupings such as those sold under the RAIPORE tradename by RAI Research Corporation.

The catholyte can be any suitable aqueous solution, including neutral or alkaline salt solutions, and any appropriate acids such as hydrochloric, sulfuric, phosphoric, nitric, acetic or others.

In a preferred embodiment, deionized or softened water or sodium hydroxide solution is used as the catholyte in the cathode compartment to produce an alkali metal hydroxide. The water selection is dependent on the desired purity of the alkali metal hydroxide by-product. The cathode compartment may also contain a strong acid cation exchange resin in a cation form such as sodium as the electrolyte.

Any suitable cathode which generates hydrogen gas may be used, including those, for example, based on nickel or its alloys, including nickel-chrome based alloys; steel, including stainless steel types 304, 316, 310, etc.; graphite, graphite felt, a multiple layered graphite cloth, a graphite cloth weave, carbon; and titanium or other valve metals as well as valve metals having coatings which can reduce the hydrogen overvoltage of the cathode. The cathode is preferably perforated to allow for suitable release of the hydrogen gas bubbles produced at the cathode particularly where the cathode is placed against the membrane.

Optionally a porous spacer material such as a chemically resistant non-conductive plastic mesh or a conductive material like graphite felt can be positioned behind the anode and/or the cathode to support the electrodes and to permit the adjustment of the gap between the electrode and the cation permeable ion exchange membrane, for example, when using high open area expanded metal electrodes. The porous spacer material preferably has large holes for ease of disengagement of the gases from the anolyte and/or catholyte. A thin protective spacer can also be placed between the anode and/or the cathode and the cation permeable ion exchange membranes. This spacer can be a non-conductive plastic or a porous conductive material like graphite felt. The cell may be operated with the electrode in contact with the thin protective spacer and the porous spacer material, or with the membrane in direct contact with the electrode and with or without the porous spacer material.

It will be recognized that other configurations of the electrolytic cell can be employed in the novel process of the present invention, including bipolar cells utilizing a solid plate type anode/cathode or bipolar membranes. For example, a bipolar electrode could include a valve metal such as titanium or niobium sheet clad to stainless steel. The valve metal side could be coated with an oxygen evolution catalyst and would serve as the anode.

An alternative anode/cathode combination which is commercially available is a platinum clad layer on stainless steel or niobium or titanium and is prepared by heat/pressure bonding.

The novel product solution contains chloric acid and alkali metal chlorate in a wide range of concentrations and ratios of chloric acid to alkali metal chlorate. For example, the solutions produced can provide molar ratios of chloric acid to alkali metal chlorate of from about 0.1:1 to about 250:1. Where the product solutions are to be used in the a known process for the generation of chlorine dioxide, suitable molar ratios of chloric acid to alkali metal chlorate of from about 0.3:1 to about 200:1, and preferably from about 1:1 to about 100:1.

These solutions are highly acidic and can be used without requiring additional acids, or where additional acids are used, permit a substantial reduction in the amount of acid employed in the generation of chlorine dioxide.

Where the solutions are to be fed to a generator for chlorine dioxide without additional acid, it is desired that ratio of total chlorate, provided by HClO₃ and an alkali metal chlorate such as NaClO₃, to the water present in the solution be greater than about 0.15. While the required concentrations of HClO₃ and, for example, NaClO₃ vary somewhat with the temperature of the solution, employing weight percent ratios of HClO₃ to NaClO₃ of about 2.7 or greater will readily provide the desired total chlorate to water ratio. Solutions of these concentrations of HClO₃ could be produced directly in the electrochemical cell. However, it may be desireable to concentrate the HClO₃ in the product solutions after removal from the cell as crystals of the alkali metal chlorate may be formed during the concentration.

The product solutions may be concentrated, for example, by evaporation at sub-atmospheric pressures and temperatures of less than about 100° C.

For example, in the range of from about 30° to about 90° C., and preferably, from about 50° to about 80° C. Solutions containing up to about 50% by weight of chloric acid, and preferably in the range of from about 30 to about 40% by weight of chloric acid, may be produced in this manner.

Freeze concentration may be used to separate sodium chlorate from the chloric acid solution and thus concentrate the remaining chloric acid simultaneously. As the solution of chlorate and chloric acid is chilled, ice and sodium chlorate will crystallize simultaneously and in separate phases. The solution will be enriched in chloric acid and reduced in water content until a eutectic point is reached at which chloric acid (hydrate) also crystalizes. This eutectic point is believed to occur at a solution concentration of about 30 to 35% chloric acid by weight. During the practice of freeze concentration it is also possible to recycle the melted ice along with the sodium chlorate, which dissolves as the ice melts for the purpose of preparing new sodium chlorate solution for feed to the process of this invention. An alternate embodiment includes a combination of freeze concentration followed by vacuum evaporation to further concentrate the chloric acid.

The novel process of the present invention permits the production of solutions having a wide range of concentrations of chloric acid and sodium chlorate for use in chlorine dioxide generators. As illustrated in FIG. 3, the product solution can be fed directly from the electrolytic cell to a commercial chlorine dioxide generator. Typical commercial processes are those which use sulfuric acid or hydrochloric acid with a reducing agent such as sulfur dioxide or methanol in the presence of a salt such as sodium chloride. Commercial chlorine dioxide processes which may use the aqueous solutions of chloric acid and alkali metal chlorate of the invention include the Mathieson, Solvay, R2, R3, R8, Kesting, SVP, and SVP/methanol, among others.

To increase yields of chlorine dioxide and conversion efficiencies it is preferred to carry out the process in the presence of a solid surface which promotes oxygen evolution. Any solid surface may be used which facilitates oxygen formation including oxygen-evolving catalysts. Suitable as oxygen-evolving surfaces or catalysts are, for example, metals and oxides of the elements of Group VIII of the periodic Table of Elements (Webster's Third New International Dictionary of the English Language. Unabridged. 1986, p. 1680). Thus metals such as the platinum group metals including platinum, palladium, iridium, rhodium or ruthenium; and mixtures or alloys of these platinum group metals may be employed. Additionally oxides of platinum group metals such as iridium, rhodium or ruthenium, as well as mixtures of these oxides with platinum group metals or alloys of these precious metals could be suitably employed. Likewise, iron alloys such as stainless steel, nickel or nickel based alloys, and cobalt based alloys can be used as oxygen-evolving catalysts in the process of the invention. Other oxygen-evolving catalysts include semiconductive ceramics known as perovskites. The catalyst may be present as particles suspended in the reaction mixture or supported on an inert substrate. The oxygen-evolving catalysts may be used in the forms of a packed bed, slurries, or any structure which will suitably promote mass transfer. In a preferred embodiment of this invention, the catalyst is supported on valve metal heat exchanger surfaces to facilitate evaporation of water during the reaction. Suitable valve metals include titanium and tantalum, among others.

The process permits flexibility in any by-product salts which may be produced as well as allowing the recovery of energy costs by producing, for example, an alkali metal hydroxide solution by-product. Further the process reduces operating costs by eliminating process steps and equipment from processes presently available.

To further illustrate the invention the following examples are provided without any intention of being limited thereby. All parts and percentages are by weight unless otherwise specified.

EXAMPLE 1

An electrochemical cell of the type shown in FIG. 1 consisting of three compartments machined from ultra high density polyethylene (UHDPE) including an anode compartment, a central ion exchange compartment, and a cathode compartment. The 1/2 inch (1.27 cm.) thick anode compartment contained a titanium mesh anode having an oxygen-evolving anode coating (PMCA 1500® Englehard Corporation, Edison, N.J.). The anode was supported and spaced apart from the UHDPE back wall using multiple layers of polyethylene mesh having 1/4 inch square holes and being 1/16 inch in thickness. A polyethylene mesh spacer was positioned between the anode and adjoining membrane to provide an anode-membrane gap of 0.0625 inch (0.1588 centimeters). The anode compartment was filled with a 2.0 percent by weight sulfuric acid solution. The 1/2 inch (1.27 cm.) thick cathode compartment contained a 304 stainless steel perforated plate cathode mounted flush to the surface of the cathode compartment with the polyethylene mesh spacers. The cathode was positioned in contact with the adjacent membrane providing a zero distance gap. The cathode compartment was initially filled with a sodium hydroxide solution (2% by weight) as the catholyte. Separating the anode compartment from the ion exchange compartment, and the ion exchange compartment from the cathode compartment were a pair of perfluorosulfonic acid cation permeable membranes with a 985 equivalent weight, obtained from the assignee of U.S. Pat. No. 4,470,888. The ion exchange compartment was a machined 1/4 inch (0.625 cm) thick frame with inlet and outlet and contained the polyethylene mesh spacers to distribute the chlorate solution as well as to support and separate the two membranes. An aqueous sodium chlorate solution containing 20 weight percent of NaClO₃ was prepared by dissolving reagent grade sodium chlorate in deionized water.

During operation of the electrolytic cell, the chlorate solution was metered into the bottom of the ion exchange compartment in a single pass process at feed rates varying from 7.0 g/min. to 14.4 g/min. Electrolyte circulation in the anode and cathode compartments was by gas lift effect only. The cell was operated employing a cell current of 24.5 amperes at a current density of 1.20 KA/m². The cell voltage varied according to the cell operating temperature. A sample of the product solution was taken at each flow rate, the temperature measured, and the product solution analyzed for chloric acid and sodium chlorate content. The product solutions were colorless, indicating no chlorine dioxide was formed in the ion exchange compartment. The concentration of the sodium hydroxide catholyte during cell operation increased to 12 percent by weight. The results are given in Table I below.

                                      TABLE I                                      __________________________________________________________________________             NaClO.sub.3 Feed                                                                            HClO.sub.3 --NaClO.sub.3 Product                          Cell                                                                               Cell                                                                               Flowrate                                                                              Product                                                                              HClO3                                                                              NaClO3                                                                              HClO3:NaClO3                                                                           Conversion                                                                            C.E.                                                                               Residence                                                                            KWH/TON                 Volts                                                                              Amps                                                                               (gm/min)                                                                              Temp (C)                                                                             Wt %                                                                               Wt % Molar Ratio                                                                            % to HClO3                                                                            %   Time (min)                                                                           of                      __________________________________________________________________________                                                            HClO3                   5.00                                                                               24.5                                                                               14.40  30.0  5.96                                                                               12.49                                                                               0.60    38.00  69.50                                                                              11.38 2082                    4.87                                                                               24.5                                                                               12.35  42.0  6.51                                                                               11.80                                                                               0.70    41.00  65.20                                                                              13.27 2152                    4.76                                                                               24.5                                                                               10.00  45.0  7.24                                                                               10.88                                                                               0.84    45.60  58.60                                                                              16.39 2336                    4.50                                                                               24.5                                                                                7.17  50.0  8.34                                                                                9.49                                                                               1.11    52.60  48.50                                                                              22.86 2674                    4.44                                                                               24.5                                                                                7.00  54.0  8.43                                                                                9.38                                                                               1.13    53.10  47.80                                                                              23.41 2673                    __________________________________________________________________________

EXAMPLE 2

The electrochemical cell of FIG. 2 was employed having a second ion exchange compartment adjacent to the first ion exchange compartment. The anode compartment containing the same type of anode used in Example 1 was filled with a strong acid hydrogen form cation exchange resin (AMBERLITE® IRC-120 plus, Rohm & Haas Company) as the electrolyte. A perfluorinated sulfonic acid-based membrane (Dupont NAFION® 417) separated the anode compartment from the first ion exchange compartment. The two ion exchange compartments were fully filled with AMBERLITE® IRC-120 plus cation exchange resin in the hydrogen form and were separated by a Dupont NAFION® 417 membrane. The same membrane was employed to separate the second ion exchange compartment from the cathode compartment. The cathode compartment contained a perforated 304 stainless steel cathode, and was filled with a sodium form AMBERLITE® IRC-120 plus cation exchange resin. Both the anode compartment and the cathode compartment were filled with deionized water. The sodium chlorate solution fed to the ion exchange compartments was prepared from reagent grade sodium chlorate dissolved in deionized water to form a 16 weight percent solution as sodium chlorate. The sodium chlorate solution at 20° C. was fed to the bottom of ion exchange compartment 20 adjacent to the cathode compartment at a flow rate of 6.5 grams per minute. The chloric acid-sodium chlorate solution flow from the upper part of ion exchange compartment 20 was routed into the bottom of ion exchange compartment 40 adjacent to the anode compartment and collected from the top of ion exchange compartment 40. The total residence time of the solution in the ion exchange compartments was about 42 minutes.

During operation of the cell, the cell current was set at a constant 23.0 amperes for an operating current density of 1.5 KA/m². The cell voltage stabilized at 9.60 volts, and the product temperature was 65° C.

Circulation in the anode and cathode compartments of the electrolyte was by gas lift effect and the liquid level of the gas disengagers was set at 3 inches (7.62 cm) above the height of the cell.

The product solution from the cell contained 11.44 weight percent as HClO₃ which represented a 90% conversion of the sodium chlorate to chloric acid. The current efficiency was determined to be 61.6% and the power consumption was 4490 KWH/Ton of HClO₃. The product solution was light yellow in color, indicating the presence of some chlorine dioxide or chlorine in the chloric acid-sodium chlorate solution product.

EXAMPLE 3

An electrochemical cell was constructed similar to that of FIG. 1 consisting of three compartments. The anolyte and catholyte compartments were machined from 1 inch (2.54 cm) thick natural polyvinylidene difluoride (PVDF). The outside dimensions of both the anolyte and catholyte compartments were 5 inches (12.7 cm) by 14 inches (35.56 cm) with machined internal dimensions of 3 inches (7.62 cm) by 12 inches (30.48 cm) by 0.250 inch (0.635 cm) deep. Flow entry and exit ports as well as flow distributions holes were drilled from the outside of the frame to the central recess area for flow into and out of the compartments.

The central ion exchanging compartment was machined from 1 inch (2.54 cm) thick natural PVDF with outside dimensions of 5 inch (12.7 cm) by 16 inch (40.64 cm) to a 1/8" (0.317 cm) by 5 inch (12.7 cm) by 14 inch (35.56 cm) thick center area with a central 3 inch (7.62 cm) by 13 inch (33.02 cm) area cutout. The 1 inch (2.54 cm) by 1 inch top and bottom ends were drilled with a single central hole to form entry/exit ports and tapped to accept 1/4 inch NPT pipe thread fittings. A series of 0.055 inch (0.1397 cm) holes were drilled every 1/2 inch (1.27 cm) from the central cutout area into the flow distribution hole in the 1 inch by 1 inch entry/exit ends of the ion exchanging compartment frame.

These flow distribution holes go through the 1/8 inch thickness of the central part of the frame.

The anolyte compartment was fitted with an open diamond pattern nonflattened expanded sheet prepared from 0.060 inch (0.1524 cm) thick titanium with the expanded metal sheet having a total depth of 0.140 inches (0.3556 cm) by 3 inch (7.62 cm) wide by 12 inch (30.48 cm) long dimensions. A 1/2 inch (1.27 cm) wide by 12 inch (30.48 cm) long by 1/16 inch (0.0625 cm) thick titanium current distributor strip was welded at multiple points to the backside of the expanded titanium sheet.

Two 1/2 inch (1.27 cm) diameter titanium current conductor posts were then welded to the back side of the flat titanium current distribution strip. The expanded titanium surfaces were then plated with a layer of metallic platinum approximately 2 micron (78 microinch) thick by a brush electroplating method using a diluted chloroplatinic acid solution. The anode structure was then mounted into the recess inside the anolyte compartment using one or more layers of 1/16 inch (0.1588 cm) thick expanded polytetrafluorethylene mesh behind the anode to make the anode surface flush with the inside surface of the anolyte compartment.

The catholyte compartment was fitted with a 1/16 inch (0.1588 cm) thick by 3 inch (7.62 cm) by 12 inch (30.48 cm) type 316L stainless steel perforated plate having two 1/2 inch (1.27 cm) diameter 316L stainless conductor posts welded on the back side. The cathode plate was mounted in the recess inside the catholyte compartment using two layers of 1/16 inch (0.1588 cm) thick expanded polytetrafluorethylene mesh behind the cathode plate to make the cathode surface flush with the inside surface of the catholyte compartment.

The electrochemical cell assembly was completed using 0.040 inch (0.1016 cm) thickness polytetrafluorethylene compressible GORE-TEX gasket tape (W. L. Gore & Associates, Elkton, Md.) on the sealing surfaces of all the compartment cell frames.

Two layers of a lose woven polytetrafluoroethylene filament with a 1/16 inch (0.1588 cm) thickness were laid in place in the central ion exchange compartment to provide for flow distribution and physical formation of the flow channel. DuPont NAFION 324 perfluorinated sulfonic acid cation permeable type membranes were then mounted between the central ion exchange compartment and the anolyte and catholyte compartments.

The above cell was operated with a reagent grade 47.15 wt % sodium chlorate solution fed at a flowrate of 11.17 gm/min into the bottom of the central ion exchange compartment. Deionized water was metered into the bottom of the catholyte compartment at a flowrate of 10.40 gm/min. The anolyte was a 30 wt % sulfuric acid which was recirculated at a flowrate of about 50 gm/min with a pump. The applied cell current was 70 amperes for a current density of 3 KA/m2 and the cell voltage was 5.55 volts. A slightly yellow product solution exited the central ion exchanging compartment at a temperature of about 80° C. and, upon analysis, had a composition of about 18.17 wt % HClO₃, 26.47 wt % NaClO₃, (total as ClO₃ ⁻, 38.71 wt %) and HClO₄ content of less than 0.03 wt %. The catholyte product contained 7.33 wt % NaOH at an output flowrate of about 12.14 gm/min. The calculated cell operating current efficiency based on the NaOH produced was 51.1%, and 51.7% based on the HClO₃ present in the product.

Chloric acid/sodium chlorate solution products from the above cell during several runs provided a composite solution containing about 20.0 wt % HClO₃ and about 22 wt % NaClO₃, [equivalent to a total chlorate ion (ClO₃ ⁻) content of about 37.0 wt %]. The total chlorate (ClO3⁻) content was determined by a standard iodiometric method using concentrated HCl and titration with thiosulfate.

The perchloric acid (HClO₄) concentration was determined by ion chromatography using dissolved sodium perchlorate solutions as standards. The sodium chlorate content of the samples were calculated by difference.

About 500 ml of the above clear, water color solution was placed in a 1 liter vacuum filter flask fitted with a thermometer, polyfluoroethylene coated magnetic stirring bar, and a 28 mm Hg vacuum water aspirator source. The flask was then placed on a hot plate/magnetic stirrer and heated while the vacuum was applied. The solution was vacuum evaporated in a temperature range between 50-80° C. Approximately 50 to 100 ml samples of the solution were periodically removed during the vacuum evaporation process. The first three samples showed no precipitates in the hot solution on sampling . The fourth and final sample was obtained when a significant amount of sodium chlorate crystals had accumulated in the vacuum flask at 60 C and there was a slight yellow color in the solution phase.

All of the samples were then cooled in stoppered flasks in a cold water bath to 20° C. for a period of 1.5 hours to allow the solution to come to equilibrium with the precipitated NaClO₃ salt phase. Samples of the water colored supernatants were then removed for analysis. The chloric acid (HClO₃) content was determined by titration with NaOH.

The results ar listed in Table II below.

                                      TABLE II                                     __________________________________________________________________________     Sample                                                                              HClO.sub.3                                                                         ClO.sub.3.sup.-                                                                     NaClO.sub.3                                                                         HClO.sub.4                                                                          ClO.sub.3 --/H.sub.2 O                                                                ClO.sub.3 /H.sub.2 O                            No.  Wt %                                                                               wt % wt % wt % wt % Ratio                                                                            Molar Ratio                                     __________________________________________________________________________     1    21.33                                                                              37.82                                                                               21.36                                                                               <0.03                                                                               0.660  0.142                                           2    25.26                                                                              38.42                                                                               17.17                                                                               <0.03                                                                               0.667  0.144                                           3    29.75                                                                              40.04                                                                               13.57                                                                               <0.03                                                                               0.706  0.152                                           4    35.85                                                                              42.73                                                                               9.32 <0.03                                                                               0.779  0.168                                           __________________________________________________________________________      Note:                                                                          Ion Chromatography Detection Limit for HClO.sub.4 was 0.03 wt %.         

EXAMPLE 4

The apparatus of Example 3 was used to produce about 150 gm of a HClO₃ /NaClO₃ solution having a composition of about 19.24 wt % HClO₃ and 27.64NaClO₃ with a total ClO₃ ⁻ content of 40.68 wt %. The solution was evaporated at temperatures between 40-60° C. under a vacuum of 28-30 mm Hg . Sodium chlorate crystals precipitated in the solution in significant amounts after 1/3 of the solution had been evaporated. The solution phase had a slight yellow color toward the end of the hot evaporation and disappeared upon cooling the solution to room temperature with applied vacuum. The solution phase from the flask was decanted from the solids and cooled in a stoppered flask in an ice bath to 5° C. The solution phase was then decanted from the sodium chlorate crystal solids and analyzed.

The solution phase contained 43.02wt % as HClO₃ and 45.24wt % as ClO₃ ⁻. This calculates to about 3.48 wt % as NaClO₃. Ion chromatography analysis showed that the HClO₄ content of the solution was <0.03 wt %. A 2.025 gm sample of the product solution was carefully evaporated to dryness on a hot plate to about 250° C. for an actual sodium chlorate residual analysis check. The final weight of the sodium chlorate deposit residual in the sample was found to be 0.065 gm or about 3.21 wt % as NaClO₃. This is in close agreement with the calculated 3.48 wt % value.

COMPARATIVE EXAMPLE A

About 150.00 gm of a solution containing 19.24 wt % HClO₃ and 27.64 wt % NaClO₃, as was used in Example 4, was evaporated by boiling in a 250 ml beaker on a hot plate. The solution boiled at an increasing value from an initial 105-107° C. to about 115° C. at the end of the run. After boiling the solution had a deep yellow color and the final weight of the solution was 116.35 gm.

After cooling to room temperature, sodium chlorate solids precipitated. About 46.88 gm of liquid was decanted from the sodium chlorate solids. The liquid decant was then further cooled in an ice bath to 5° C. The clear decant was analyzed and found to contain about 27.88 wt % HClO₃ and 1.66 wt % HClO₄ by ion chromatography. The residual NaClO₃ content of about 13.47 wt % was determined by evaporating a 2.005 gm sample of the liquid to obtain 0.270 gm of solids.

EXAMPLE 5

About 300 ml of HClO₃ /NaClO₃ product solution having a composition of about 19.24wt % HClO₃ and 27.64% NaClO₃ with a total 40.68 wt % ClO₃ ⁻ were evaporated at 28-30 mm Hg vacuum at temperatures between 40-60° C. Liquid samples of the evaporated solution were taken from the flask during the evaporation process. In examples 5-8 the solution was cooled to 15° C.; in example 9 the solution was cooled to 5° C.; and in example 10 the solution was cooled to 0° C. After cooling, the solution phase was decanted from the precipitated sodium chlorate crystals and analyzed.

The results, given in Table IIIA below, show that the sodium ion i.e. Na+ content, in the chloric acid can be significantly reduced to levels below 1% as Na+. The highest chloric acid concentration achieved in was 51.39 wt %.

Table IIIB below shows the published solubility limits of sodium chlorate at 15°, 20° and 25° C. As can be seen, at 15° C., a saturated sodium chlorate solution has a ClO₃ ⁻ /H₂ O molar ratio of 0.155. As shown by examples 2, 3, 4, 5 and 6 in Table IIIA, HClO₃ /NaClO₃ product solutions of the invention are produced having a higher ClO₃ ⁻ /H₂ O molar ratio than that of the saturated sodium chlorate solution.

Thus, the advantage with the HClO₃ /NaClO₃ solutions of the invention having ClO₃ ⁻ /H₂ O ratios greater than about 0.155 is that there is more chlorate ion (ClO₃ ⁻) available for chlorine dioxide production per weight or volume of solution than in the typically used commercial sodium chlorate solutions, as well as less sodium ion and less water.

                                      TABLE IIIA                                   __________________________________________________________________________     Example                                                                             Solution                                                                            HClO3                                                                              ClO3.sup.-                                                                         NaClO3                                                                              HClO4                                                                              Na.sup.+                                                                           HClO3                                                                               ClO3.sup.-                                                                          ClO3.sup.- /H2O                                                                       ClO3.sup.- /H2O                No.  Sp. Gr.                                                                             Wt %                                                                               Wt %                                                                               Wt % Wt %                                                                               Wt %                                                                               Molarity                                                                            Molarity                                                                            Wt Ratio                                                                              Molar Ratio                    __________________________________________________________________________     5*   1.3230                                                                              20.50                                                                              37.74                                                                              22.30                                                                               <0.03                                                                              4.82                                                                               3.21 5.98 0.660  0.142                          6*   1.3161                                                                              32.65                                                                              41.14                                                                              11.33                                                                               <0.03                                                                              2.45                                                                               5.09 6.49 0.734  0.159                          7*   1.3309                                                                              35.29                                                                              42.70                                                                              9.99 <0.03                                                                              2.16                                                                               5.56 6.81 0.780  0.168                          8*   1.3474                                                                              42.04                                                                              46.52                                                                              6.35 <0.03                                                                              1.37                                                                               6.71 7.51 0.901  0.195                          9**  --   42.72                                                                              45.30                                                                              2.58 --  0.56                                                                               --   --   0.828  0.179                          10***                                                                               --   51.39                                                                              52.50                                                                              1.42 --  0.31                                                                               --   --   1.113  0.240                          __________________________________________________________________________      *Decanted product solution cooled to 15° C.                             **Decanted product solution cooled to 5° C.                             ***Decanted product solution cooled to 0° C.                      

                                      TABLE IIIB                                   __________________________________________________________________________     PUBLISHED SOLUBILITY OF SODIUM CHLORATE:                                                Solution                                                                            NaClO3                                                                              ClO3.sup.-                                                                         ClO3.sup.-                                                                          ClO3.sup.- H2O                                                                        ClO3.sup.- H2O                              Temperature °C.                                                                  Sp. Gr.                                                                             Wt % Wt %                                                                               Molarity                                                                            Wt Ratio                                                                              Molar Ratio                                 __________________________________________________________________________     15       1.4060                                                                              47.80                                                                               37.50                                                                              6.314                                                                               0.718  0.155                                       20       --   48.90                                                                               38.30                                                                              --   0.750  0.162                                       25       1.4240                                                                              50.00                                                                               39.20                                                                              6.689                                                                               0.784  0.169                                       __________________________________________________________________________ 

What is claimed is:
 1. A process for producing an aqueous solution of chloric acid and alkali metal chlorate in an electrolytic cell having an anode compartment, a cathode compartment, and at least one ion exchange compartment between the anode compartment and the cathode compartment, which comprises feeding an aqueous solution of an alkali metal chlorate to the ion exchange compartment, electrolyzing an anolyte in the anode compartment to generate hydrogen ions, passing the hydrogen ions from the anode compartment through a cation exchange membrane into the ion exchange compartment to displace alkali metal ions and produce an aqueous solution of chloric acid and alkali metal chlorate, passing alkali metal ions from the ion exchange compartment into the cathode compartment, removing the aqueous solution of chloric acid and alkali metal chlorate from the ion exchange compartment, and, increasing the chlorate ion concentration of the aqueous solution of chloric acid and alkali metal chlorate to provide a total chlorate ion to water molar ratio of about 0.15 or greater.
 2. The process of claim 1 accomplished by evaporating the aqueous solution of chloric acid and alkali metal chlorate at sub-atmospheric pressures and temperatures of less than about 100° C.
 3. The process of claim 2 accomplished by maintaining the temperature of the evaporation in the range of from about 30° to about 90° C.
 4. The process of claim 1 accomplished by maintaining the chloric acid concentration in the aqueous solution of chloric acid and alkali metal chlorate at about 30% by weight or higher.
 5. The process of claim 1 accomplished by cooling the aqueous solution of chloric acid and alkali metal chlorate to form sodium chlorate crystals and ice crystals in a solution of chloric acid containing dissolved alkali metal chlorate.
 6. The process of claim 5 accomplished by separating the sodium chlorate crystals and the ice crystals from the solution of chloric acid containing dissolved alkali metal chlorate.
 7. The process of claim 6 accomplished by evaporating the aqueous solution of chloric acid containing dissolved alkali metal chlorate at sub-atmospheric pressures and temperatures of less than about 100° C. 